Rhodium alkyne-coupling reactions

A tetrahydrofuran fused with a seven-membered ring was obtained from an enyne through a [5+2] cycloaddition reaction catalyzed by [(C10H8)Rh(COD)]+ SbF6 complex <02AG(E)4550>. Rhodium-catalyzed carbonylative alkene-alkyne coupling reactions... 

Shibata et al. reported that, in the presence of an iridium catalyst, the carbonylative alkyne—alkyne coupling reaction of the diyne 102 with carbon monoxide gave the tetrahydrofuran-fused cyclopen-tadienone 103 (Scheme 35).114 The rhodium-catalyzed alkyne—alkyne coupling reaction of 102 with the isocyanide 104 produced the iminocyclopentadiene 105 (Scheme 36).114b These reactions proceed through formation of the metallacyclopentadiene intermediate 106, which undergoes insertion either of CO or of the isocyanide 104. 

A similar coupling reaction of salicyl aldehydes with disubstituted alkynes, catalyzed by rhodium, is known... 

Using established principles of late-transition metal catalysis, several research groups have engineered multi-component coupling reactions from the basis set of known Group 9 metal vinylidene-mediated reactions. In 2004, Jun and coworkers described a new method for the synthesis of enones via rhodium vinylidene-mediated hydrative dimerization of alkynes (Table 9.12) [24]. 

As far as alkynes are concerned, C-C coupling reactions have been reported to be catalyzed by rhodium or ruthenium clusters. Phenylacetylen can be coupled to diisopropyl carbodiimide to give 93... 

Work on mechanistic details is in progress, but the present transformation may result from a catalytic cycle that involves the transmetalation between the hydroxo-rhodium(I) 4 and arylboronic acid to give the arylrhodium(I) species 5, and the insertion of enone to the Ar-Rh bond. The hydrolysis of the rhodium(I) enolate with water reproduces the hydroxorhodium species, as shown in Figure 1. The arylrhodium(I) complexes are unstable such as to preclude isolation in pure form, but they have been reasonably speculated to be the key intermediates carrying out various coupling reactions with organic halides and the addition to alkenes and alkynes. 

The catalytic C-C coupling of alkynes has been widely reported for rhodium and ruthenium complexes (e.g., see [113-120]) however, examples of iridium catalysts are less frequent [121-123]. The low activity observed for Ir complexes could be attributed to the greater tendency of rhodium and ruthenium to form vinyUdene complexes [124—127], since it is generally accepted that the formatiOTi of Z-enynes occurs via vinyUdene intermediates [117-120]. In this regard, the intermetalUc cooperation makes it possible to form the Ir-vinylidene intermediates required for the formation of Z-enynes and, ultimately, C-C coupling reactions. 

In 2006, the group of Artok showed that 5-aryl-2(5H)-furanones could be prepared in moderate to good yields by a rhodium-catalyzed carbonylative arylation of internal alkynes with aryl boronic acids (Scheme 1.9a) [22]. a,P-Unsaturated ketones (chal-cone derivatives) were formed as the major product when some TFA (trifluoroacetic acid) was added under the same reaction conditions [23a]. By varying the catalytic system, indanones could be produced as the main product [23b]. The chemical behavior of terminal alkynes is different, and either a,P-unsaturated ketones or furans starting from propargylic alcohols can be achieved (Scheme 1.9b) [24, 25]. In the case of vinyl ketones, 1,4-diketones were obtained by rhodium-catalyzed coupling of arylboronic acids in the presence of 20-40 bar of CO [26]. In 2007, Chatani demonstrated that indenones could be accessed by a carbonylative rhodium-catalyzed cyclization of alkynes with 2-bromophenylboronic adds (Scheme 1.9c) [27]. Here, the key intermediate is a vinylrhodium(I) spedes that is formed by transmetaUation of RhCl with 2-bromophenylboronic acid followed by insertion of... 

Shinokubo and coworkers [79] performed a domino coupling reaction of aryl boronic acids 139 with internal alkynes 140 and acrylates 141 by using a rhodium catalyst in water to give nine examples of the 1,3-diene derivatives 142 in 37-81% yields (Scheme 12.55). The reaction indicates that the use of water offers the possibility to optimize known reactions, leading to novel transformations. 

A rhodium-catalyzed protocol of alkyne cyclooligomerization provides an alternative route for the preparation of indenocorannulenes 100 [27] (Scheme 29). The key step of the synthesis starts with 2,3-diethynylcoranulenes 99, which are accessed by Pd-catalyzed cross-coupling reactions of the corresponding 2,3-dichlorocorannulenes 19/23 with trimethyltin-substimted alkynes. The... 

Rhodium(lll)-catalyzed redox—neutral coupling of N-phenoxyacet-amides and alkynes led to benzo[l)]furan derivatives (13AGE6033). Furo[2,3-l)]pyran-6-one derivatives were prepared via rhodium(II)-catalyzed reactions of diazo compounds and ethynyl compounds (13T9294). Copper-mediated oxidative annulation of phenols and unactivated internal alkynes afforded benzo[l ]furan derivatives (13CS3706). E t-kaurane maoecrystalV was produced via C-H functionalization (13JA14552). Rhodium-catalyzed intramolecular C-H... 

Polysubstituted 2E/-pyran-2-ones are available through rhodium(III)-medi-ated decarboxylative and dehydrogenative coupling reactions of maleic acid derivatives with alkynes, in moderate to excellent yields (Scheme 45) (13JOC11427). 

Decarbonylation of the acylmetal intermediates has been extended to a variety of carbon-carbon forming reactions. For example, Li et al. reported decarbonylative addition reactions of aromatic aldehydes to alkynes (Scheme 7.17) [24] and acrylates [25]. An oxidative decarbonylative coupling reaction of aromatic aldehydes with 2-arylpyridines was promoted by a rhodium catalyst (Scheme 7.18) [26]. Carboxylic acid derivatives were also employed for analogous carbon-carbon bond-forming reactions through decarbonylation [27]. 

The same ruthenium(II) catalytic system with 1 equiv. of Cu(0Ac)2-H20 oxidant was used to generate 2-pyridones directly firom acrylamides by C-H and N-H bond functionalization and annulation with alkynes. This reaction offers improved substrate scope with respect to the similar reaction reported with rhodium catalyst [(Eq. 88)] [177]. The reaction is applicable to dialkylacetylenes. Alkylphenyla-cetylenes lead to regioselective annulation with an (aryl)C-N linkage formation, consistent with the coupling of the electron-deficient alkyne carbon with the electron-rich carbon of the Ru-C bond. 

A -Benzyloxindoles and Af-benzyldihydroisoquinolinones were synthesized via rhodium(I)-catalyzed [2- -2-1-2] cycloaddition of 1,6-diynes with alkynes, palladium(0)-catalyzed arylative cyclization, and palladium(0)-catalyzed coupling reaction with arylboron or aryltin reagents (Scheme 4.37) [41]. When the reactions were conducted under a CO atmosphere, aryl ketones were obtained through... 

Coupling reactions of aromatic, heteroaromatic, and vinylic substrates with alkynes via sp C—H bond cleavage under rhodium, iridium, ruthenium, palladium, or nickel catalysis to produce a wide range of cyclic compounds have been described in this chapter. These annulation reactions provide powerful methods for constructing a variety of -rr-conjugated molecules containing fused aromatic and heteroaromatic nuclei from simple, readily available substrates. Extensive efforts will continue to be made to extend the scope of starting materials for this catalysis. 

In 2007, the Mlura group reported a Rh(III)-catalyzed oxidative coupling of benzoic acids with internal alkynes to the synthesis of isocoumarins via aromatic C-H activation (Scheme 6.24a) [38]. Importantly, the reaction of benzoic acids with alkynes takes place efficiently even with a reduced amount (5mol%) of Cu(0Ac)2-H20 under air (Scheme 6.24b) [5b]. The same group also developed the rhodium-catalyzed coupling of acrylic acids with alkynes to provide corresponding a-pyrone via vinylic C-H bond cleavage (Scheme 6.24c) [5c]. In 2015, Wen and coworkers described a Rh(III)-catalyzed synthesis of... 

The rhodium-entrapped cage compound which is formed using a stoichiometric amount of [RhCl(CO)2]2 is a notable paradigm of the rhodium-catalyzed [2-I-2-I-1] al-kyne-alkyne and CO coupling [35]. Heating 57 in acetone at 50 °C for 8 h or irradiation by a tungsten or mercury lamp provided the cage compound in 50% yield based on NMR spectroscopy. However, due to mechanical losses it was isolated in only 16% yield from the reaction mixture, by crystallization as the hexafluorophosphate salt 58 (Eq. 13). 

The Lee group originated rhodium alkenylidene-mediated catalysis by combining acetylide/alkenylidene interconversion with known metal vinylidene functionalization reactions [31], Thus, the first all-intramolecular three-component coupling between alkyl iodides, alkynes, and olefins was realized (Scheme 9.17). Prior to their work, such tandem reaction sequences required several distinct chemical operations. The optimized reaction conditions are identical to those of their original two-component cycloisomerization of enynes (see Section 9.2.2, Equation 9.1) except for the addition of an external base (Et3N). Various substituted [4.3.0]-bicyclononene derivatives were synthesized under mild conditions. Oxacycles and azacycles were also formed. The use of DMF as a solvent proved essential reactions in THF afforded only enyne cycloisomerization products, leaving the alkyl iodide moiety intact. 

While platinum and rhodium are predominantly used as efficient catalysts in the hydrosilylation and cobalt group complexes are used in the reactions of silicon compounds with carbon monooxide, in the last couple of years the chemistry of ruthenium complexes has progressed significantly and plays a crucial role in catalysis of these types of processes (e.g., dehydrogenative silylation, hydrosilylation and silylformylation of alkynes, carbonylation and carbocyclisation of silicon substrates). 

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